We propose to continue studies on the structure, required amino acid residues, and function of tryptophanyl-tRNA synthetase of Escherichia coli. By DNA sequencing of the hybrid plasmid in the superproducing strain we have determined the enzyme primary structure. The E. coli enzyme is extensively homologous to the cognate enzyme of Bacillus stearothermophilus and bears limited homology to the human placental enzyme. Amino acid modification studies have implicated cysteine residues 70 and 97 and lysine residues 111 and 113 in enzyme activity. These residues are near or in the bridge peptide between the amino terminal one-third and carboxy-terminal two-thirds of the enzyme. We shall explore the roles of these and other residues more fully.